For words that have only three letters, there is not any 4th letter. As an example with the word “increase”, there are no extra letters afterwards and thus must be the word "increase". The word “addict” is during the BIP39 wordlist however , you would have applied “addi” for “addict”. https://fatallisto.com/story8797388/how-much-you-need-to-expect-you-ll-pay-for-a-good-bip39-definition